Files released under the 30 year rule revealed the former Prime Minister was warned to break ties with an adviser who claimed to have masterminded the defeat of the miners in the bitter year long strike from 1984 to 1985.

Those close to Margaret Thatcher warned that unless she severed her links with David Hart - a wealthy Old Etonian property developer - he would end up causing her “grave embarrassment”.

From his suite at Claridges in London, the flamboyant Mr Hart established himself as a go-between for Mrs Thatcher and National Coal Board chairman Ian MacGregor and working miners, making regular forays to the coalfields in his chauffeur-driven Mercedes.

He was said to have bankrolled the breakaway Union of Democratic Mineworkers and organised the legal action by working miners which led to the strike by Arthur Scargill’s National Union of Mineworkers being ruled illegal.

Mr Hart later boasted Mrs Thatcher came to rely on him completely, claiming: “It got to the point where she really let me run it.”

But the files have now revealed that by the time the strike was drawing to a close in 1985 there was mounting concern in Downing Street about Hart’s activities.

In February 1985 Mrs Thatcher’s political secretary Stephen Sherbourne wrote: “Though DH has on occasions provided you with useful intelligence he has recently been pursuing his own ends at the expense of those of the Government.

“DH has his own views on how the coal strike should end and has been pursuing his cause even when it conflicted with the interests of yourself and (Energy Secretary) Peter Walker. And in so doing he has exploited his No 10 connection.”

He added: “So long as he feels he can telephone me regularly on whatever issue, there will be a risk of grave embarrassment to you.”

The link between the pair was abruptly broken shortly afterwards when a misjudged attempt by Mr Hart to lobby the Americans on behalf of a British defence supplier resulted in the contract they were seeking being awarded to the French.

Mr Hart nevertheless re-emerged in the 1990s as an adviser to Conservative defence secretaries Malcolm Rifkind and Michael Portillo.